OK, well there are several issues here.
Number one, your floor has no thickness, it is just a flat plane without depth. So you can’t use Push/Pull.
Secondly, you have an odd grouping structure with elements nested inside other elements and none of them named. That will likely lead to confusion. This is where Outliner comes in useful as you can see the structure.
I recommend that you start by simplifying the nested groups. Then, when you have the floor as a distinct group, go into it and give the floor some thickness using Push Pull. Then draw a rectangle on one surface representing the stairwell and Push Pull to create the opening. Then close the group and get rid of what you have drawn above the floor (which presumably does not represent anything).
And get used to naming your groups so they make sense when reading Outliner. Some folk don’t use groups and only use components. When you create a component SU encourages you to name it (though it does offer a generic default name). That seems like a bore at first but it only takes a moment (if you keep it simple but unique) and you soon get used to doing it.
Just noticed another issue. Your floor is not flat. Look at the Z axis dimensions on this screenshot. This will create all kinds of problems unless it was intentional.